Gelatin silver halide compositions and elements containing a water-soluble hydroxyalkyl ether derivative of starch



United States Patent 3,152,906 GELATIN SILVER HALEDE COMPQSETEONS ANDELEMENTS CQNTAINING A WATER-SQLUBLE HYDROXYALKYL ETEER DERIVATIVE 0FSTARCH Joseph De Witt Overman, New ghrewsbury, N.J., assignor to E. I.du Pont de Nemours and Company, Wilmington, DeL, a corporation ofDelaware No Drawing. Filed May 7, 1962, Ser. No. 193,653 15 Claims. (Ct.%94) This invention relates to photographic compositions and tophotographic elements including the same. More particularly, it relatesto such compositions and elements having improved photographiccharacteristics, especially increased covering power of developed silverand improved maximum density, contrast and effective speed.

.It is known to add certain chemical compounds, e.g., heavy metal andnoble metal salts, to improve the sensitometric characteristics ofphotographic gelatino-silver halide emulsions. There is, however, at thepresent time, a practical limit to the advantages which can be obtainedby the addition of such compounds without accompanying deleteriouseffects such as uncontrollable fog and instability on tropical aging.

It is also known to add to, or replace all or part or" the gelatin in agelatin-silver halide system with various polymeric colloid materialsfor various reasons including attempts to overcome the well-knowndisadvantages of gelatin. However, none of these proposed materials hasovercome these disadvantages in a completely satisfactory manner, andgelatin is still used in most commercial photographic films. In general,when all of the gelatin is replaced by the synthetic polymers suggestedby the prior art, the outstanding photographic and colloid-chemicalproperties of gelatin are lost. In addition, it is difficult todetermine the optimum conditions for coating and drying, etc. to form aphotographic element possessing satisfactory hardness and stability.This is particularly true when the exposed element contains the usualphotographic hardeners and is placed in aqueous photographic processingsolutions.

It is, therefore, an object of this invention to provide photographiccompositions and elements having improved covering power of thedeveloped silver. Another object is to provide such compositions andelements which. also have improved maximum density, contrast andeffective speed. A further object is to provide such improvedcompositions and elements wherein apart of the gelatin is replaced witha relatively inexpensivernaterial: Yet another object is to provide suchimproved compositions and elements without an accompanying loss of othervaluable properties, Other objects will appear hereinafter.

These and other objects are accomplished in accordance with thisinvention by the use of a photographic composition having improvedcovering power of developed silver comprising a gelatino-silver halideemulsion containing from about 20 to 80 parts by weight, preferably 30to 70 parts, per 100 parts by Weight of gelatin present in saidemulsion, of a water-soluble hydroxyalkyl ether derivative of starchhaving an average molecular weight of from about 10,000 to 60,000wherein from about 10 to 20% of the hydroxyl groups of said starch havebeen ,ethe'rified to hydroxyl lower alkyl ether groups.

\ based on the American Standard Method for the Sensi 3,152,906 PatentedGet. 13, T964 ice In general, the above-described etherified starchderivative willvconstitute from about 5,to 50 percent and preferablyfrom about 7 to 25 percent, by weight of total solids in thegelatino-silver halide emulsion. The emulsions may, of course, containsmall amounts of conventional adjuvants.

The hydroxyalkyl ether derivatives of starch can be prepared by thegeneral method of Hjermstad as set forth in Whistler, Industrial Gums,chapter XXXI, pp. 727-740 (Academic Press, New York, 1959). When themolecular weight of the starch derivative substantially exceeds 60,000the material tends to become incompatible with gelatin. The hydroxyethyland hydroxypropyl ether derivatives are particularly preferred.

Quite unexpectedly, it is found that the final emulsions of thisinvention have markedly increased covering power (e.g., 10 to 60%greater) over emulsions containing gelatin alone. More specifically, itis found that large increases in maximum density and contrast can beobtained in the developed image without any increase in the quantity ofsilver halide present; Covering power can be expressed as the numericalresult of dividing optical density by the grams of silver per squaredecimeter in the developed image layer.

The covering power will, of course, vary with the amount of theetherified starch material used.

The invention is especially suited to photographic silver halideemulsions for radiological films, particularly those for medicaldiagnostic work. However, the invention is by no means limited to anyparticular type of photographic emulsion, and the etheriiied starchmaterial may be utilized in any gelatino-silver halide system to improveits efiiciency. The invention is also very useful in the so-calledgraphic arts films, i.e., lithographic films. In all cases when one ofthe hydroxyalkyl ethers of starch is present in the amounts set forthabove, a higher density is obtained from a given amount of metallicsilver in the developed photographic layer. Although the effect is morenoticeable in-large-grain emulsions, it is also effective in smml-grainemulsions.

The methods of preparation of the compositions of this invention arequite simple, and consist of merely admixing with the molten orliquified emulsion after the digestion step, an aqueous solution ofthehydroxyalkyl ether of starch. As an exemplary procedure, agelatinosilver iodobromide emulsion is prepared by conventional methods.The silver halides are precipitated in gelatin and the resultingdispersion or emulsion ripened. The emulsion is then washed, eitherafter chilling and noodling, or after coagulation by decanting thesupernatant liquid. The emulsion is then redispersed and then it isdigested to bring it to maximum speed. Following digestion, theesterified starch material (usually in aqueous solution) is added to themolten emulsion in an amount sufiicient to provide the desired ratio ofetherified starch to total solids. The usual final additions, e.g., ofhardener, antifogging agents, sensitizing agents and spreading agents,are made and the emulsion is coated on a suitable support and dried inthe usual manner. To test the eifectiveness of the hydroxyalkyl ether ofstarch, the element is exposed in 'a sensitometer according to .aprocedure tometry of Medical X-Ray Films PH2.9-1956. F

-field viscosimeter.

3 The sensitorneter usedlin the following examples was equipped with aneutral density /2 step Wedge. The density of a selected step, as setforth in the tables, was measured ad the quantity of metallic silver persquare decimeter was determined by analysis. The density di- EXAMPLE I Awheat starch is selected of which about 20% of the hydroxyl groups havebeen etherified with hydroxypropyl groups and a 10% aqueous solution ofwhich has a viscosity at 25 C. of 25 cps, as measured by a Brook- Anaqueous solution of the latter is added to a high speed gelatino-silveriodobromide emulsion which is made in the usual manner and digested withan organic sulfur compound and gold chloride. The

emulsion contains approximately 1.6 mole percent of 'silver iodide and98.4 mole percent of silver bromide.

The emulsion is then divided into three portions and enough of theetherified starch material described above is added to each to give theparts per 100 parts of gelatin in the emulsion shown in the Table Ibelow. An unaltered portion of the emulsion serves as a control. Theemulsions are coated and dried in the conventional manner. and thecoatings are exposed in the sensitometer and developedfor 5 minutes in adeveloper of the composition: I

' Gram p-N-methylaminophenol hydrosulfate 3 Anhydrous sodium sulfite 50Hydroquinone 9 Anhydrous sodium carbonate 50 Potassium bromide 4.5

Water to make 1000.0 ml.

Following development the films are fixed, washed and dried in theconventional manner. Representative results of sensitometric tests andmeasurements of covering power are shown in the following table:

Example I is repeated except that the etherified starch .is added in anamount of 61 parts per 100 parts of gelatin The hydroxypropyl ether ofstarch described in Example I is added in the. amounts set forth inTable III below to the gelatino-silver iodobromide emulsion made asdescribed in Example I. The emulsions are coated and dried in theconventionalmanner and the coated films are exposed and processed asdescribed above. The following results are representative:

Table 111 Parts oi Starch Covering Film No. Derivative 'Maximum Power atper 100 Density Maximum Parts of Density Gelatin It has beenfound thatthe increased covering power of photographic emulsion layers is notlimited to adding the above-described etherified starches to thegelatinosilver halide emulsion layer. It can be attained'by in-.corporating the materials in a composition to be coated next to agelatino-silver'halide emulsion layer, e.g., in a gelatin sublayer, aseparator or light-filtering layer or in an antiabrasion layer. Theamount of etherified starch material so used can be based on the totalgelatin in the contiguous layers. A layer essentially composed of one ora mixture ofthe hydroxyalkyl ethers of starch,e.g., 75% to 100%composition, can be used in a-layer contiguous with-agelatino-silverhalide emulsion layer.

The hydroxyalkyl ethers of starch are commercially available andmay beobtained from the Hercules PowderCom'pany, Wilmington, Delaware.

This invention is not limited to photographic gelatinosilver halideemulsions of silver iodobromide type; it may be applied to othergelatino-silver halide emulsions, e.g., gelatino-silver bromo-chlorideemulsions of the lithographic type. The invention is particularlyefi'icacious in photographic emulsions Whose average grain size isrelatively large. The emulsions may contain any of the wellknown opticalsensitizingdyes as well as non-optical sensitizers such as sulphursensitizers, e.g., allyl isothiocyanate, allyl diethyl thiourea, phenylisothiocyanate and sodium thiosulfate; the polyoxyalkylene ethersdisclosed in U.S. Patent 2,400,532 and the polyglycols disclosed in U.S.Patent 2,423,549. Other non-optical sensitizers such as the aminesdisclosed in U.S. Patent 1,925,508 and the metal salts disclosed in U.S.Patents 2,540,085 and 2,540,- 086 may also be used. Antifoggants, e.g.,benzothiazole and triazaindenes can'be used as well as the usual hardtothe silver iodobromide emulstion defined. in Example I and coated anddried, exposed and processed as described therein with the followingrepresentative results:

7 Table II Parts of I Starch Covering Film'No. Derivative Maxim Power atper 100 Density Maximum Parts of Density Gelatin .1 (Control) 0 1.37 .30

e'ners, i.e., chrome alum, formaldehyde, etc. The emulsions may becoated on any suitable support suchas paper or films composed ofcellulosic esters, e.g., cellulose tri- -acetate, celluloseacetate/butyrate; superpolymers, e.g.,

vinyl chloride/vinyl acetate copolymers, polyvinyl acetals, e.g.formaland acetal; polystyrene; polyamides, e.g., polyhexamethylene adipamide,and polyesters, e.g., polyethylene terephth'alate, polyethyleneterephthalate/isophthalatef and those .-terephthalic acid or polyestersobtainable by condensing dimethyl terephthalate with propyleneglycoLdiethylene glycol, tetramethylene glycol or cyclohexane 1,4dimethanol (hexahydro p xylene alcohol). The vinylidene chloridecopolymer-coated oriented polyesters films of'USJPatents 2,627,086 and2,779,684 areespecially suitable.

An advantage of the invention is that it'provides a simple, dependableand eifective means for. providing gelatino-silver halide photographicemulsions of enhanced covering power. The efiiciency of the resultingdeveloped silverv is increased, that is, it canprovide greater densityper quantity'of metallic silver which results from development.

Another advantage of the invention is that the addition of thehydroxyalkyl ethers of starch to photographic emulsions requires nospecial technique and can be carried out by the ordinary technician withconventional apparatus. Since the compounds are water soluble they canbe added from an aqueous solution and they do not provide the problem ofsolvent recovery. Also, the resulting modified gelatino-silver halideemulsions can be coated and dried in the conventional coating and dryingapparatus which ofiers commercial advantages. It is, therefore, apparentfrom the foregoing description of the invention that the objects thereofhave been achieved as well as still further advantages apparent to thoseskilled in the art.

What is claimed is:

1. A photographic composition having improved covering power ofdeveloped silver comprising a gelatinosilver halide emulsion containingfrom about 20 to 80 parts by weight, per 100 parts by weight of gelatinpresent in said emulsion, of a water-soluble hydroxyalkyl etherderivative of starch having an average molecular weight of from about10,000 to 60,000 wherein from about to of the hydroXyl groups of saidstarch have been etherified to hydroxy lower alkyl ether groups.

2. A photographic composition according to claim 1 wherein said emulsioncontains from about 30 to 70 parts of said water-soluble hydroxyalkylether derivative of starch per 100 parts of gelatin.

3. A photographic emulsion according to claim 1 wherein said hydroxylower alkyl ether groups are hydroxypropyl ether groups.

4. A photographic emulsion according to claim 1 wherein said hydroxylower alkyl ether groups are hydroXyethyl ether groups.

5.A photographic element having improved covering power of developedsilver comprising a support layer and a gelatino-silver halide emulsionlayer, said silver halide having in intimate contact therewith fromabout 20 to 80 parts by weight, per 100 parts of gelatin present in saidelement, of a water-soluble hydroxyalkyl ether derivative of starchhaving an average molecular weight of from about 10,000 to 60,000wherein from about 10 to 20% of the hydroxyl groups of said starch havebeen etherified to hydroxy lower alkyl ether groups.

6. A photographic element having improved covering power of developedsilver comprising a support layer and a gelatino-silver halide emulsionlayer containing from about 20 to 80 parts by weight, per 100 parts ofgelatin present in said element, of a water-soluble hydroxyalkyl etherderivative of starch having an average molecular weight of from about10,000 to 60,000 wherein from about 10 to 20% of the hydroxyl groups ofsaid starch have been etherified to hydroxy lower alkyl ether groups.

7. A photographic element having improved covering power of developedsilver comprising a support layer, a gelatino-silver halide emulsionlayer, and a gelatin-containing layer contiguous to said emulsion layer,said gelatin-containing layer containing from about 20 to 80 parts byweight, per 100 parts of gelatin present in said element, of awater-soluble hydroxyalkyl ether derivative of starch having an averagemolecular weight of from about 10,000 to 60,000 wherein from about 10 to20% of the hydroxyl groups of said starch have been etherified tohydroxy lower alkyl ether groups.

8. A photographic element according to claim 5 wherein the amount ofsaid water-soluble hydroxy alkyl ether derivative of starch is from 30to parts per 100 parts of gelatin.

9. A photographic element according to claim 5 wherein said hydroxylower alkyl ether groups are hydroxypropyl ether groups.

10. A photographic element according to claim 5 wherein said hydroxylower alkyl ether groups are hydroxyethyl ether groups.

11. A process for preparing a gelatino-silver halide photographiccomposition having improved covering power of developed silver whichcomprises admixing with said composition from 20 to parts per parts ofgelatin of a water-soluble hydroxyalkyl ether derivative of starchhaving an average molecular weight of from about 10,000 to 60,000wherein from about 10 to 25% of the hydroxyl groups of said starch havebeen etherilied to hydroxy lower alkyl ether groups.

12. A process for preparing a gelatino-silver halide photographicelement of a gelatino-silver halide emulsion on a support whereby saidelement has improved covering power of developed silver which processcomprises incorporating in intimate contact with said emulsion 20 to 80parts per 100 parts of gelatin in said emulsion of a water-solublehydroxyalkyl ether derivative of starch having an average molecularweight of from about 10,000 to 60,000 wherein from about 10 to 20% ofthe hydroxyl groups of said starch have been etherified to hydroxy loweralkyl ether groups.

13. A process according to claim 11 wherein the amount of saidwater-soluble hydroxy alkyl ether derivative of starch is from 30 to 70parts per 100 parts of gelatin.

14. A process according to claim 11 wherein said hydroxy lower alkylether groups are hydroxypropyl ether groups.

15. A process according to claim 11 wherein said hydroxy lower alkylether groups are hydroxyethyl ether groups.

References Cited in the file of this patent UNITED STATES PATENTS3,043,691 Weyde July 10, 1962 3,043,698 Chambers July 10, 1962 3,047,392Scott July 31, 1962 3,063,838 Jennings Nov. 13, 1962 3,067,033 WeydeDec. 4, 1962 3,069,267 Chambers Dec. 18, 1962 3,085,009 Chambers Apr. 9,1963 3,085,010 Chambers Apr. 9', 1963 3,087,818 Chambers Apr. 30, 1963UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No., 3,152,906 October 13 1964 Joseph De Witt Overman It is hereby certifiedthat error appears in the above numbered patient requiringicorrectionand that the said Letters Patent should read as corrected below.

Column 6, line 24, for "25%" read 20% 7 Signed and sealed this 9th dayof March 1965.

(SEAL) Attest:

EDWARD J. BRENNER ERNEST W. SWI'DER' Attesting Officer Commissioner ofPatents

1. A PHOTOGRAPHIC COMPOSITION HAVING IMPROVED COVERING POWER OFDEVELOPED SILVER COMPRISING A GELATINOSILVER HALIDE EMULSION CONTAININGFROM ABOUT 20 TO 80 PARTS BY WEIGHT, PER 100 PARTWS BY WEIGHT OFGELATING PRESENT IN SAID EMULSION, OF A WATER-SOLUBLE HYDROXYALKYL ETHERDERIVATIVE OF STARCH HAVING AN AVERAGE MOLECULAR WEIGHT OF FROM ABOUT10,000 TO 60,000 WHEREIN FROM ABOUT 10 TO 20% OF THE HYDROXYL GROUPS OFSAID STARCH HAVE BEEN ETHERIFIED TO HYDROXY LOWER ALKYL ETHER GROUPS.